The present invention relates to a laser oscillation wavelength monitoring device which is in particular used at an optical fiber communication system utilizing a wavelength division multiplex (WDM) technology that implements optical transmission by multiplexing wavelengths of laser light sources of plural different wavelengths.
At an optical transmission system, a WDM, which transmits plural wavelength light in one string of optical fiber, has been utilized. In this WDM application, as light sources, plural different wavelength laser light sources are needed.
In order to keep good transmission quality, it is necessary that the oscillation wavelengths are controlled in high accuracy, at these laser light sources.
At the control for the laser oscillation wavelength, a wavelength monitoring device using a dielectric multi layer film is used. Japanese Patent Application Laid-Open No. HEI 10-9961 discloses a wavelength monitoring device, which uses a dielectric multi layer film. FIG. 1 is a diagram showing the construction of this conventional wavelength monitoring device.
In this conventional wavelength monitoring device, a transmitted light through an optical band pass filter 24 (transmission center wavelength=monitoring wavelength xe2x80x9cxcexaxe2x80x9d), which is made of a dielectric multi layer film, is received at a photodiode 26 and a change of an oscillation wavelength of a laser light source 10 is monitored by a change of photo current. That is, the light inputted to an optical transmission line 12 from the laser light source 10 branches at directional couplers 14 and 20, and the light outputted from the directional coupler 14 is converted to an electric signal by a photodiode 16 and logarithmically amplified at a logarithmic amplifier 18. The light outputted from the directional coupler 20 is inputted to the photodiode 26 through the optical band pass filter 24. The optical band pass filter 24 is an optical element whose transmittance factor decreases corresponding to that a wavelength is shifted off the specific wavelength xe2x80x9cxcexaxe2x80x9d and the light outputted from the optical band pass filter 24 is converted to an electric signal by the photodiode 26 and logarithmically amplified at a logarithmic amplifier 28. A differential amplifier 30 outputs the difference between outputs of the logarithmic amplifiers 18 and 28, and a comparator 32 compares the output of the differential amplifier 30 with a threshold value Vref of an alarm, and at the time when the output of the differential amplifier 30 is more than the threshold value Vref, the comparator 32 outputs an alarm signal.
However, at the mentioned above conventional wavelength monitoring device, the transmission light intensity at the dielectric multi layer film is changed by a polarization state of an incident light. That is, there exists a polarization dependency, it is impossible to judge whether the change of the photo current is caused by the change of a laser oscillation wavelength or the change of the inputted polarization, therefore there is a problem that a wavelength monitoring in high accuracy is impossible.
It is therefore an object of the present invention to provide a laser oscillation wavelength monitoring device which can obtain a changing direction and a changing amount of the oscillation wavelength electrically by not affected by the polarization state of the incident light and can be easily manufactured.
According to the present invention, for achieving the object, a laser oscillation wavelength monitoring device provides a splitting means for splitting an incident light, which is inputted from an optical transmission line in which an optical signal from a laser light source is transmitted, into different polarization states, plural optical/electric converting means for converting the optical signals of each split polarization state to electric signals respectively, and an accumulating means for accumulating the ratios of said electric signals expressing the light intensity of said polarization states from said each optical/electric converting means, every designated unit time, designated times, and monitors a change of a laser oscillation wavelength of unit time, by compensating the change of the polarization state of said incident light, with averaging by time the ratios of the light intensity of the polarization states.
According to the present invention, a laser oscillation wavelength monitoring device provides an optical lens for converting an incident light, which is inputted from an optical transmission line in which an optical signal from a laser light source is transmitted, to a beam, a polarizer which makes an arbitrary linear polarized light in said beam transmit, an optical band pass filter means through which only a specific wavelength band light of said arbitrary linear polarized light is transmitted, a polarization splitting means for splitting the light transmitted through said optical band pass filter means into a first polarization state and a second polarization state, a first and a second optical/electric converting means for receiving said each polarization state split at said polarization splitting means respectively and converting said polarization states to electric signals, and an outputting means which accumulates the ratios of output levels of said first and second optical/electric converting means every designated unit time and averages the accumulated results of designated times and outputs the averaged result.
According to the present invention, by installing a laser oscillation monitoring device of the present invention in an optical fiber line, the laser oscillation wavelength is monitored without any relation with a polarization state of an incident light and a changing direction and a changing amount of the oscillation wavelength can be electrically obtained.